Cystic fibrosis (CF) is a genetic disorder characterized by chronic respiratory infections and pancreatic insufficiency. Growth fialure occurs in approximately 25% of patients with CF and is associated with more severe respiratory disease and decreased survival. Successful nutritional rehabilitation has been shown to improve respiratory status, decrease hospitalizations for pulmonary exacerbations and improve energy level. Megestrol acetate is a synthetic progestational hormone that has been shown to increase appetite and promote weight gain in patients with HIV/AIDS and certain forms of cancer. At the UAB CF Center 5 patients (3 females and 2 males; 12-28 years) received megestrol acetate for CF related growth failure. All 5 subjects have had a significant increase in appetite and associated weight gain, 3.3 to 8.4 kg (5-20% above pretreatment weight). All patients had an incrase in energy level, an increase in daily activity level and improved body image. Side effects were minor and resolved with decreasing megestrol acetate dose. We propose to evalute megestrol acetate in a randomized, double-blinded placebo controlled study to examine the effectiveness and safety of megestrol acetate therapy in patients with CF related growth failure. Primary outcome measurement will be weight gain; secondary outcome measurements will include caloric intake, body composition, quality of well being and pulmonary function.